1. Field of the Invention
The present invention relates to a scanning optical module designed to emit a scanning beam onto a surface of an image bearing member, and an image forming apparatus equipped with the scanning optical module.
2. Description of the Related Art
An electrophotographic process is employed by most image forming apparatuses, and a scanning optical module for use therein is provided with a polygon mirror. A scanning beam from a light source is scanningly deflected in a given direction by the polygon mirror, and emitted onto an electrostatically pre-charged surface of an image bearing member such as a photosensitive drum, whereby an electrostatic latent image is formed on the surface of the photosensitive drum. Then, the electrostatic latent image is developed using toner to form a toner image. This toner image is transferred onto a sheet and then fixed thereon.
During the above operation, the polygon mirror is drivenly rotated according to rotation of a driving motor, and the rotation of the driving motor inevitably gives rise to heat generation in a motor shaft. As measures against this heat, there has been known a technique of absorbing the heat by a heat sink, and air-cooling the heat sink by a cooling fan (see, for example, JP 06-015870A and JP 2000-206627A).
Along with advances in performance of image forming apparatuses, there is an increasing need for drivenly rotating the polygon mirror at a higher speed. Under such a high-speed rotation, a significantly large amount of heat is generated in the motor shaft. In this case, an optimal cooling for the heat sink is achieved by allowing an airstream from the fan to be distributed all over the heat sink.
The above conventional technique is designed such that an airstream from the fan is simply supplied toward the heat sink. In this cooling approach, the airstream is liable to reach only a portion of the heat sink, resulting in failing to achieve the optimal cooling and causing a problem about deterioration in reliability of the scanning optical module. Specifically, in conjunction with heat generation in the motor shaft, a housing for the polygon mirror is expandingly deformed. The expansive deformation leads to image quality defects (e.g., a focus defect and a magnification defect due to displacement), because even a slight displacement of a scanning beam-emitting position relative to the photosensitive drum has a direct impact on image quality. As above, in terms of positively cooling the heat sink, there remains a problem in the conventional technique.